.box_3 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  background-image: url(../oneemoji.jpg);
  animation: bgsize 5s infinite;
  background-size: 100px 100px;
}

@keyframes bgsize {
  50% {
    background-size: 500px 500px;
  }
}